1. Technical Field
The present invention relates to the field of arc discharge lamps, and in particular to a method of stabilizing arc attachment in arc discharge lamps.
2. Description of Related Art
High pressure discharge lamps generate light by passing an electrical current from one electrode to another electrode through a metal vapor sealed inside a discharge vessel to form an arc between the electrodes. Discharge lamps include a discharge chamber or vessel 10 contained within an outer bulb 12, as illustrated in FIG. 1. The discharge vessel 10 typically includes a cathode 14, an anode 16, a starting gas 18 and a metal. When an electric field is passed between the cathode 14 and anode 16, the starting gas 18 ionizes, which decreases the resistance between the electrodes 14, 16 and creates an arc between them. The charged arc emits visible light and ultraviolet light when excited electrons return to lower orbitals.
High pressure discharge lamps typically utilize mercury or other various emission metals and halogens (metal halides) to enhance the light output and brightness. A short arc type discharge lamp may comprise a mercury discharge lamp, a metal halide lamp or another high pressure discharge lamp in which the distance between the two electrodes where the arc is established is relative small, e.g. approximately 5 mm or less. Short arc mercury lamps are often used in the photochemical industrial field, semiconductor device manufacturing field, projector field and the like. In such discharge lamps, a coil 20 serves as the starting point of discharge at startup time of lamp and is fixed on the starting electrode 14 so as to contact the side surface of the electrode 14. During startup of this kind of short arc type discharge lamp, when a power supply applies an electrical current across the lamp electrodes 12, 14, typically a glow discharge is started between the anode 16 and the coil 20 fixed to the side surface of the cathode 14, and gradually shifts to an arc discharge. The heat of the coil 20 heated by the discharge is conducted to the cathode 14 to which the coil 20 is fixed. Furthermore, the cathode 14 is subjected to radiation heat due to the arc from the coil 20, which leads to the state in which thermoelectrons are easily emitted. As the internal pressure of the lamp increases, the arc ideally narrows down to be stable and the stabilized arc discharge ideally shifts to the tip of the cathode 14 to generate a stationary lit up state of the lamp.
However, with the above-mentioned kind of short arc type discharge lamp, a problem arises that the arc discharge commenced from the coil 20 at startup time often becomes stabilized on the coil 20 and does not shift to the tip of the cathode 14. This kind of ‘arc-hangup’ phenomenon noticeably occurs during the initial low pressure stages during start up of the lamp, where the present inventors have found that approximately 80% of short arc type discharge lamps have the arc become stabilized on the coil 20. This ‘arc-hangup’ phenomenon tends to occur more frequently at lower pressures, because at higher pressures the arc will tend to seek the shortest distance between the electrodes, namely the distance between the electrode tips. This phenomenon is potentially thought to occur because the heat produced from the arc and emitted by the coil 20 is transferred from the coil 20 to the rear end part of the cathode 14 which makes contact with the coil 20, and, because additional thermal conduction to the bulb 12, the occurrence of thermoelectron emission at the tip of the electrode becomes difficult and whereby the arc remains on the coil 20 without being transferred to the tip of the electrode.
The ‘arc-hangup’ phenomenon produces an abnormal discharge that often causes the arc to contact the outer wall of the lamp and create problems such as lamp explosion, cloudiness of the lamp, or blackening of the lamp due to the vaporization of the coil 20 due to abnormal heating of the coil 20. Moreover, the length of the arc is much longer than the intended arc gap between the tips of the cathode 14 and anode 16, which makes the lamp unusable for most optical applications. These problems associated with such abnormal discharge negatively impact the intended properties of the discharge lamp and essentially render the lamp unusable.
Prior attempts to solve this ‘arc-hangup’ phenomenon have centered around changing the shape or design of the electrodes to promote an arc that extends between the electrode tips or by driving the lamp with a higher in-rush current to more greatly warm the electrodes. However, these solutions tend to overpower the electrodes and can cause them to wear out more quickly, aside from also presenting design limitations on discharge lamp manufacturers by requiring specific cathode shapes that may promote a stable tip-to-tip arc.